feat: dump current config over serial on READ_CONFIG (#16)

Add a serial command so the configuration tool can import the keymap
currently on the device. When the serial input is "READ_CONFIG", print
the current keyconfig.json wrapped in <<<CONFIG_BEGIN>>> / <<<CONFIG_END>>>
markers instead of treating it as an incoming config to save. The existing
write path (send JSON to update) is unchanged.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: DriftKingTW

src/main.cpp

@@ -720,6 +720,19 @@ void loop() {
     // Accept Serial input for keyconfig.json
     if (Serial.available() > 0) {
         String jsonString = Serial.readString();
+        jsonString.trim();
+
+        // Config read request: dump the current keyconfig.json (wrapped in
+        // markers) so the configuration tool can import what's on the device.
+        // Emit as a single write to minimize interleaving with Serial output
+        // from tasks running on the other core.
+        if (jsonString == "READ_CONFIG") {
+            Serial.print("\n<<<CONFIG_BEGIN>>>\n" +
+                         loadJSONFileAsString("keyconfig") +
+                         "\n<<<CONFIG_END>>>\n");
+            return;
+        }
+
         Serial.println("Received JSON:");
         Serial.println(jsonString);